I used this factory Bushmaster with Armalite mid-length barrel in Andy Stanford's Tactical Carbine course earlier this summer without a hitch:
That being said I have been to several Tactical Carbine Courses, Patrol Carine Courses, SWAT Carbine course, shoot 3 gun matches from time to time, etc, etc, etc.
I would have had problems in the class had I not done my 1,000 round break in period first. During this break in period I realize at around 600 round that the screws for the bolt carrier key were not properly staked and backed out causing the rifle to short stroke.
I am one of the Colt belivers, but feel that other makes will do fine as well. My experience has shown me that a factory Colt tends to have less problems than other makes. The problem with the carrier key was the 2nd new factory Bushmaster that I've had the same problem with.
I would highly recommend a factory gun from RRA, Colt, or Bushmaster with a chrome lined barrel. Learn how to maintian and lube your rilfe properly, and break it in with 1,000 to 2,000 rounds of good quality factory ammo prior to the class. If there are problem with your new gun they will show themselves during the break in period (most of the time).
Anything that you bolt onto the gun (ie. Aimpoint with mount, etc) use Loc-Tite BLUE on ALL the bolts of the mount.....Anything that I bolt on a gun from a pistol grip, to a new buttstock, to optics, etc. I use Loc-Tite BLUE on to keep the bolts from backing out.
Take care and have fun in the class.
Semper Fi
Jeff